The Turonian-Coniacian boundary in Ollogoyen (Basque Country); foraminifers and nannoplankton

Abstract

The Ollogoyen Section (Basque Country, Northern Spain) shows a pelitic sequence which is rich in several fossil groups. We have studied the foraminifers and coccoliths taken from more than 40 samples along a 65 m succession of marls and marly limestones including the Turonian-Coniacian boundary. They are rich in microfossils and the observed differences in numbers are in direct relation to the preservation. Unfortunately, the recorded benthonic foraminifers and nannoplankton do not show any index fossils for the Tu/Co boundary. On the other hand, planktonic foraminifers show several biostratigraphical markers. Among the globigerinids we have used the appearance of the species Hedbergella flandrini Porthault, Archaeoglobigerina cretacea (d'Orbigny) and A. blowi Pessagno. Among the globotrunacanids, the species Marginotrucana sinuosa Porthault and M. schneegansi s.s. (Sigal) are useful in placing the Tu/Co boundary. Heterohelicids show a morphological sequence from the biserial Heterohelix pulchra (Brotzen), of late Turonian age, and the multilocular specimens of the genus Ventilabrella, of early Coniacian age. This determination of the Turonian-Coniacian boundary is backed up by the associated microfauna and microflora which is found between the last known Romaniceras spp. specimens and the earliest appearance of Coniacian ammonites. This enables us to mark the Tu/Co boundary in Ollogoyen with the appearance of the planktonic foraminiferal species M. sinuoso, M. schneegasi s.s., and ? Ventilabrella sp.